Recorded in several forms including MacGilfoyle, Gilfoyle, and Kilfoyle, this is an Irish surname. It originates from the pre 10th century Gaelic name Mac Gilla Phoil, meaning "The son of the follower of St. Paul". It is said that some nameholders in Ireland called Powell derive from the same source, but this is difficult to quantify as the English and Welsh surname Powell is also well recorded in Ireland. The clan were once the chiefs of Kings County, which since the establishment of the Republic has been renamed Offaly. Their castle which seems to have entirely disappeared, was in the village of Shinrone. Quite why the first named holder was so renowned for his devotion to St Paul as to be named after him is unclear, but a number of Irish surnames do originate from association with particular saints or holy men, taken from the early days of Christianity. In Elizabethan times two brothers Nichol and Owen MacGilfoil are recorded on the land charters of Sir William O'Karrell of Tipperary in 1576, whilst in the infamous Potato Famine registers of 1846 - 1848, Betsy Kilfoyle appears on the passenger list of the ship "Jane of Liverpool", which left for New York on May 5th 1846.